1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly, to a semiconductor device having a fuse element which enables a change of a circuit configuration through cutting of the fuse element.
2. Description of the Related Art
In a manufacturing process for semiconductor devices, there has been a method of changing a circuit configuration by cutting, after a wafer manufacturing process is finished, a fuse element made of, for example, polysilicon or metal, with the use of a laser, for example. With this method, after the electrical characteristics of a semiconductor device are measured, the value of a resistance can be corrected to obtain desired characteristics. Accordingly, this method is an effective method particularly in completion of semiconductor devices whose analog characteristics are important. In the method using a laser, stably cut of a fuse element is required.
In Japanese Patent Application Laid-open No. 9-199596, there is proposed a method involving an increase in an area of a fuse element which is fused by a laser or a formation of a constricted fusing part to gain a power margin, thereby stably cutting the fuse element.
Further, in Japanese Patent Application Laid-open No. 9-36234, there is provided a method involving a connection of stacked fuse elements with a contact, and cutting of the contact portion by a laser such that cutting failure hardly occurs.
One of problems in cutting a fuse element by a laser is cutting failure. As illustrated in FIG. 4, when fuse elements 15 are cut by a laser, a film which forms the fuse element 15 is blown off and a fuse cut mark 16 is formed. At this time, there occurs, for example, a short-circuit due to a fuse remain 17 that is formed when the fuse element 15 is not completely melted and evaporated due to lack of energy, or a short-circuit due to a fuse readhesion 18 that is an adhesion of a fuse element once melted and evaporated to its surrounding, both resulting in cutting failure. Short-circuit due to the fuse readhesion 18 may occur between adjacent fuse elements 15, and the situation is schematically illustrated in FIG. 4.
Those types of cutting failure are more liable to occur when a thicker film is formed on the fuse element 15. For example, in a case of a multilayer wiring structure using a polysilicon layer located in a lower layer portion as fuse elements 15, films formed on the fuse elements 15 vary widely, and it is difficult to obtain a condition that enables more stable fuse cutting. This means that the thicknesses of the films remaining in an opening region 43 of the protective film vary. The opening region 43 of the protective film is formed above the fuse element 15, and corresponds to a region obtained by removing a final protective film, for example, a nitride film.
In Japanese Patent Application Laid-open No. 9-199596, which is an example of the related art, it is conceivable that there is a risk of a short-circuit when the fuse readhesion 18, illustrated in FIG. 4, occurs since the cut surfaces of the fuse elements 15 are exposed at the same layer.
In Japanese Patent Application Laid-open No. 9-36234, which is another example of the related art, the fuse elements are stacked and the fuse element located in the upper layer has the constricted part such that the risk of a short-circuit, which may occur in Japanese Patent Application Laid-open No. 9-199596, is reduced. In this structure, however, the fuse element located in the lower layer is irradiated with a laser to no small extent. When the fuse element located in the lower layer is concerned, there is a high risk that the fuse element does not completely evaporate or readheres to its surrounding since the film formed on the fuse element is thick, and the fuse element does not receive sufficient energy from the laser. As a result, short-circuit between the fuse element located in the upper layer and the fuse element located in the lower layer may occur due to the fuse readhesion 18.